You can include content from private GitHub projects / repos too. Once private links are added, the OAuth process will follow.
Include GitHub for Confluence Cloud provides a File Macro
To add the GitHub Cloud macros for Confluence Cloud to a page:
In the Confluence editor, choose Insert > Other Macros.
Find and select the required macro ("External Content" category).
Speeding up macro input with auto-complete: Type /GitHub or /Include to see a list of suggested macros.
To edit an existing macro: Click the macro placeholder and choose Edit. A macro dialog window will open, where you can edit the parameters of the GitLab Cloud macro.
1. Macro "GitHub File": Include GitHub Cloud file by url
This macro allows the user to include a file from a GitHub repository in order to have an up-to-date version of it in a Confluence page.
Type /GitHub File and ENTER to specify the details of this macro like the url of the file, syntax highlighting, title and line numbers.
Note: Copy the file URL from your browser and paste it into the URL field.
Alternatively, you can also paste a GitHub file URL directly into the Confluence page editor.